Scanning laser spectrometer

Abstract

A scanning spectrometer incorporating a scanning diode laser powered with a controllable injection current. The injection current control may be set at predetermined discrete levels. These levels may be varied in accordance with signals derived from a servo loop, and may have superimposed upon them a cyclically varying substantially constant amplitude current. The laser output is directed via a beam splitter to both sample and reference cells. The material in the reference cell is so selected as to provide absorption features at each of the desired frequencies. The cyclically varying current is provided with an amplitude sufficient to cause a frequency amplitude at least as great as the maximum breadth of each of these spectral features of interest. The preselected bias currents are selected so as to provide lasing action of the diode at or near each of the selected frequencies. A phase-locked amplifier tuned to the frequency of the cyclically varying current senses the position of the scanned laser output relative to that of the associated absorption feature and provides an error signal to adjust the bias current so as to center the cyclically scanned laser frequency on the selected absorption feature.

What is claimed is: 
     
       1. A spectrometer system comprising in combination: a semiconductor diode laser capable of emitting energy at an output frequency variable in response to a varying injection current;   temperature control means including means for maintaining said laser at a predetermined temperature;   optical means including a beam divider for apportioning selected energy from said laser along a plurality of optical paths;   first detection means disposed with respect to one of said paths for detecting said energy propagating along said one path;   second detection means disposed with respect to another of said paths for detecting said energy propagating along said another path;   means for disposing an absorbing reference material along said one path between said first detection means and said laser so as to attenuate a portion of said laser energy propagating along said one path, said reference material having selected absorption features of determinable spectral widths at optical frequencies of interest;   means for disposing a sample in said another optical path between said second detection means and said laser so as to intercept another portion of said laser energy propagating along said second path;   a plurality of amplifying means connected to said first and second detectors for providing electrical output signals proportional to the laser energy incident on the respective detectors;   laser control means for controlling said injection current of said laser, said laser control means including: (a) means for providing substantially constant currents at a plurality of predetermined values corresponding to injection currents suitable to cause said laser to emit said energy at discrete frequencies substantially corresponding to said selected absorption features,   (b) means for cyclically varying said constant currents about said predetermined values by at least one predetermined amplitude at at least one predetermined frequency, said predetermined amplitude corresponding to a range of injection currents corresponding to a range of said output frequencies at least as large as a selected spectral width of one of said absorption features, and   (c) means for varying said constant currents in response to an error signal,   said amplifying means including phase detection means to determine the phase relationship between said means for cyclically varying and at least one said electrical output signal, said means for phase detection providing said error signal.     
     
     
       2. A spectrometer according to claim 1 and further including system control means including means for accumulating and comparing said electrical output signals responsively to one of said predetermined values of current so as to generate a sample-related value to a predetermined level of accuracy, and thereafter for commanding said laser control means to provide said substantially constant current at another of said preselected values.